Optical scale and optical encoder using same

ABSTRACT

When a convex lens  13  comes close to 0-th order transmitted light T 0  with rotation of an optical scale  11 , the 0-th order transmitted light T 0  derived from a light beam L emitted from a light source  15  and transmitted through a diffraction grating  12  around is converged and incident on a reference position signal detection sensor  14 . At that time, a large output is obtained from the reference position signal detection sensor  14 . This signal constitutes a reference position signal. On the other hand, reflective diffracted light beams La, Lb having been reflected by the diffraction grating are detected by a diffraction light detection sensor  16  after interference, so that the speed and the angle etc. of the scale  11  are detected. In this way, the reference position signal and an incremental signal are detected at the same position on the optical scale.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scale that detects therelative angle and relative displacement between a scale and detectionmeans. The present invention also relates to an optical encoder usingthe same.

2. Related Background Art

Rotary encoders and linear encoders have been used as angle detectionsensors or position detection sensors. In particular, in the case whereposition detection with high resolution is required, an incrementaloptical encoder is used.

The incremental encoder is a measuring device for detecting a relativeangle or a relative displacement by counting two incremental signals, asdisclosed in Japanese Patent Application Laid-Open Nos. 2000-266567 and2003-97975.

However, what can be detected by counting incremental signals is arelative displacement. Therefore, to enable position detection, areference position signal related to an external coordinate system isneeded.

It is possible to obtain such a reference position signal using separateexternal means.

On the other hand, there has been known an encoder that is adapted tooutput a datum position signal at a specific position in addition toincremental signals to provide a reference signal by the encoder itself.This datum signal is sometimes referred to as a reference signal or anoriginal point signal.

SUMMARY OF THE INVENTION

Position information with high accuracy is desired in high resolutionencoders.

To realize highly accurate position detection, it is necessary to detecta reference position signal with high accuracy.

In a optical scale of a linear encoder, a pattern 3 for generating areference position signal is typically provided in the vicinity of anincremental pattern 2 of the optical scale 1 as shown in FIG. 9.

The patterns are illuminated with a light beam L having a diameter largeenough to cover the patterns 2 and 3 as shown in FIGS. 10 and 11.

By detecting it using an independent detection system 4, a referenceposition signal is obtained from the reference position signalgenerating pattern 3.

These patterns 2 and 3 can be irradiated by light beams from differentlight sources to optimize the optical system. However, such a structureis hardly adopted, since the light source is a relatively expensivepart.

A part of a light beam directed to the incremental pattern 2 may besplit so as to be used in generating reference position signal. However,such an arrangement leads to complication of the structure.

In the case where a reference position signal is detected by the systemshown in FIGS. 9 to 11, if the optical scale that should be properly atthe position shown in FIG. 12A is displaced by its movement in a shiftor angular direction different from that to be detected as shown in FIG.12B, the phases of a reference position signal and incremental signalsobtained by a reference position signal detection sensor S and twoincremental signal detection sensors Sa, Sb respectively will change.

FIG. 12B shows a case where the optical scale 1 has been displaced in arotational direction. In the case of a rotary encoder, a change in thephase also occurs if the rotation center is displaced.

To reduce such displacement, the detection position of the incrementalsignal and the detection position of the reference position signal aregenerally arranged close to each other. However, a small displacementsometimes matters in cases where high accuracy in detection is required.

The above-described angular displacement of the optical scale 1 isgenerated due to inclination in terms of precision in a linear movementguide with respect to linear movement, and due to fluctuation of therotation center in terms of precision in rotary shaft with respect torotational movement. Even if there are such variations in the posture,detection results of the reference position signal and the twoincremental signals will be always the same in the case where thevariations have positional repeatability (repeatable variations).However, variations that do not have positional repeatability(non-repeatable variations) occur in reality, which will cause theabove-mentioned error. In this case, position information represented byone of the two incremental signals and position information representedby the other incremental signal are different from each other, and it isnecessary to determined which signal is the correct signal to be used.

An object of the present invention is to provide an optical scale thatcan generate a reference position signal as well as two incrementalsignals with high accuracy using the same light source.

To achieve the above object, according to the present invention, thereis provided an optical scale for outputting an incremental signal and areference position signal technically characterized in that a patternfor generating an incremental signal and a pattern for generating areference position signal are formed on a substrate, and unnecessarytransmitted light or unnecessary reflected light that is not used ingenerating the incremental signal, derived from a light beam having beenincident on the pattern for generating the incremental signal, is madeincident on the pattern for generating the reference position signalthat is provided coaxially.

In the optical scale according to the present invention, it is possibleto detect the reference signal and the incremental signal at the sameposition, and therefore relationship between the phases of theincremental signal and the reference position signal is not influencedby changes in the posture of the optical scale. Accordingly, positiondetection and angle detection with high accuracy can be made possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an optical scale according to a firstembodiment.

FIG. 2 illustrates light beams transmitted and reflected by adiffraction grating.

FIG. 3 schematically shows the optical scale.

FIG. 4 illustrates a method of detecting a reference position signal.

FIG. 5 schematically shows an optical scale according to a secondembodiment.

FIG. 6 schematically shows an optical scale according to a thirdembodiment.

FIG. 7 shows light path of a light beam reflected by an diffractiongrating.

FIG. 8 shows light path of a light beam transmitted through andiffraction grating.

FIG. 9 is a plan view of a conventional optical scale.

FIG. 10 is a plan view of the conventional optical scale in a state inwhich it is irradiated with a light beam.

FIG. 11 is a side view of the conventional optical scale in a state inwhich it is irradiated with a light beam.

FIGS. 12A and 12B illustrate how output signals changes with changes inthe posture of the conventional optical scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail based on embodimentsshown in FIGS. 1 to 8.

First Embodiment

In the first embodiment, an optical scale 11 has a main body composed,for example, of a disk-like glass substrate as shown in FIG. 1. On theoptical scale 11, a reflective diffraction grating 12 with a constantpitch made of a metal film such as aluminum or chrome is annularlyformed along the circumference of the optical scale 11. In addition, aconvex lens 13 is provided on the backside of the optical scale 11 at aportion corresponding to the diffraction grating 12.

In the case where the diffraction grating 12 is an amplitude gratingconstituted by presence and absence of a reflective film, when a lightbeam L is projected onto it from above as shown in FIG. 2, 0-th orderreflected light R0, plus 1st order reflected light R+1, minus firstorder reflected light R−1, 0-th order transmitted light T0, plus 1storder transmitted light T+1 and minus 1st order transmitted light T−1are generated in addition to reflective diffracted light andtransmissive diffracted light.

FIG. 3 is a schematic partial view showing a part of the scale 11. Areference position signal detection sensor 14 composed, for example, ofa photo diode is disposed below the optical scale 11. The convex lens 13is used as a pattern for generating a reference position signal with the0-th order transmitted light T0, and the diffraction grating 12 is usedas a pattern for generating incremental signals.

A light beam L is incident on the diffraction grating 12, and resultantreflective diffracted light beams La and Lb are caused to interfere byan interference optical unit not shown. Thus, two incremental signalsare generated. On the other hand, the 0-th order transmitted light T0transmitted through the optical scale 11 and converged by the lens 13 isincident on the reference position signal detection sensor 14, whereby areference position signal is generated.

FIG. 4 illustrates a method of detecting the reference position signal.The light quantity of the 0-th order transmitted light T0 derived fromthe light beam emitted from the light source 15 and transmitted throughthe diffraction grating 12 is constant if the structure and the materialof the optical scale 11 is uniform. Therefore, the output of thereference position signal detection sensor 14 that detects the 0-thorder transmitted light T0 is constant as long as the convex lens 13 isnot present. When the convex lens 13 comes across the 0-th ordertransmitted light T0 with rotation of the optical scale 11, the lightaround it is converged and incident on the reference position signaldetection sensor 14. This results in a increased output of the referenceposition signal detection sensor 14. This signal constitutes thereference position signal.

On the other hand, the reflective diffracted light beams La, Lb aredetected by a diffracted light detection sensor 16 after interferencewith each other, whereby the speed and the angle etc. of the scale 11are detected.

As per the above, in the first embodiment, it is not necessary to splitout a light beam for generating a reference position signal.Accordingly, the structure of the detection optical unit can be madesimple. Moreover, it is not necessary to use such a large diameter lightbeam with which both a diffraction grating for generating theincremental patterns and a convex lens as the pattern for generating thereference position signal are illuminated, and therefore the light beamL is utilized efficiently. Thus, the quality of the incremental signalsis enhanced.

In addition, since the convex lens 13 is disposed coaxially with theillumination light for generating the incremental signals, relationshipof the phases of the incremental signals and the reference positionsignal will not change with a change in the posture of the optical scale11.

There is, for example, a high resolution encoder using a diffractiongrating in which an incremental signal is obtained by interference ofdiffracted light beams. In this case, however, unnecessary 0-th orderlight and transmitted or reflected light are generated by thediffraction grating that serves as a signal for generating incrementalsignals.

In the first embodiment however, it is possible to detect the referenceposition signal at the position same as the pattern used for detectingincremental signals by illuminating an appropriate reference positionpattern detection optical unit with 0-th order transmitted light T0.

Second Embodiment

FIG. 5 schematically shows the second embodiment. In this optical scale21, a transmissive hole pattern 22 in the form of a pinhole is formed onthe glass plate. As with the first embodiment, a reference positionsignal can be obtained as an increase in the light quantity detected bythe reference position signal detection sensor 14 caused by 0-th ordertransmitted light T0 that has passed through the transmissive holepattern 22.

The shape of the transmissive hole pattern 22 is not limited to acircle, but it may be a two-dimensional pattern such as a slit.Alternatively, a light blocking pattern may be formed instead of thetransmissive hole pattern 22. In the case where a light blocking patternis used, only the sign (i.e. plus and minus) of the light quantitydetected is reversed.

Third Embodiment

FIG. 6 schematically shows the third embodiment.

On the back surface of an optical scale 31 composed of a glasssubstrate, a transmissive diffraction grating 32 serving as anincremental pattern is formed.

A convex lens 33 is formed on the upper surface of the optical scale 31.

As shown in FIG. 7, a light beam L is made incident on the diffractiongrating 32 obliquely to its grating arranged direction, and 0-th orderreflected light R0 reflected by the diffraction grating 32 is guided tothe upper surface of the optical scale 31.

When entering the convex lens 33, the 0-th order reflected light R0 isconverged onto a reference position signal detection sensor not shown,so that the reference position signal can be detected in a similarmanner as in the first embodiment.

As shown in FIG. 8, 1st order diffracted light beams derived from thelight beam L incident on the optical scale 31 and transmitted throughthe transmissive diffraction grating 32 are caused to interfere witheach other to generate incremental signals.

The transmissive diffracted light beams La, Lb that have beentransmitted through the diffraction grating 32 are made incident on aninterference optical unit, so that two incremental signals are obtained.

The convex lens 33 used in the third embodiment may be replaced by anoptical element such as a Fresnel lens.

The convex lenses 13, 33 used in the first and the third embodiment maybe replaced by a transmissive hole pattern similar to that used in thesecond embodiment, a slit or a light blocking pattern.

This application claims priority from Japanese Patent Application No.2004-338895 filed Nov. 24, 2004, which is hereby incorporated byreference herein.

1. An optical encoder for detecting relative movement of a scale unitand light projection unit, comprising: a scale unit; a light projectionunit for projecting a light beam onto said scale unit; a diffractiongrating portion formed on said scale unit and irradiated with said lightbeam; a reference position detection portion formed on said scale unitand irradiated with said light beam; a first sensor for generating anincremental signal using said light beam reflected by or transmittedthrough said diffraction grating-portion; a second sensor for generatinga reference position signal using said light beam reflected by ortransmitted through said reference position detection portion, whereinwhen one of said incremental signal and said reference position signalis generated using a transmitted light component of said light beam, theother of said incremental signal and said reference position signal isgenerated using a reflected light component of said light beam.
 2. Anoptical encoder according to claim 1, wherein said reference positiondetection portion is a transmissive hole formed on said substrate.
 3. Anoptical encoder according to claim 1, wherein said reference positiondetection portion is disposed at a position substantially the same as apart of said diffraction grating portion.
 4. An optical encoderaccording to claim 1, wherein said reference position detection portionis a lens.
 5. An optical encoder according to claim 4, wherein saidreference position detection portion is a Fresnel lens.
 6. An opticalencoder according to claim 1, wherein said diffraction grating portionand said reference position detection portion are formed on oppositesurfaces of said scale unit.
 7. An optical encoder according to claim 6,wherein said diffraction grating portion and said reference positiondetection portion have portions partly overlapping with each other atleast as seen from a direction perpendicular to the scale unit.
 8. Ascale for use in an optical encoder, comprising: a scale unit; adiffraction grating portion provided on said scale unit; a lens unitprovided on a surface of said scale unit opposite to said diffractiongrating portion, wherein said diffraction grating portion and said lensunit have portions partly overlapping with each other at least as seenfrom a direction perpendicular to the scale unit.